In my mind late 80’s and early 90’s MTBs make ideal commuting bikes for loads of various reasons .
A lot of them were available with good quality tubesets ie various grades of Tange and Reynolds spring to mind straight away never mind early 4130 cro mo that came along a bit later . All of these should provide a decent quality ride .
Most of them won’t have heavy suspension either which I always feel to be totally pointless unless you are hurtling down a bumpy dirt track instead of just trundling along a paved cycle path on your way to the office .
The majority will have at least 18 speeds and there is a good bet that the small “ Granny Ring ” won’t have seen any or had very little useage in its life time and if you are lucky they will be paddle controlled rather than twist gripped . A lot of them came with tried and tested shimano running gear as well .
The first modification to make is to throw the big fat knobbly tyres away and fit some more road friendly tyres that won’t howl once you reach a certain speed like the big open treaded knobbly ones will do . The other advantage of road friendly tyres is that they are a lot easier to keep rolling along than a set of knobbly’s. The vast if not all will be running on 26” wheels so loads of choice should new rims be required .
Another strong point is the Cantilever braking systems are normally fairly strong in standard format never mind with the adoption of some newer style brake pads like Kool stop. Okay they won’t give you the stopping power of a set of modern disc brakes but they will do the job well enough with some general servicing and up keep .
Most of the other items like seat posts, stems and bars will be alloy items not steel so again a lot lighter than their steel cousins . Some makers also did some of these components with nice colours rather than the plain old alloy or black finish . People like Girvin offered a quill stem with some suspension built in as well , these are quite sort after today .
So all in all there is no need to go and buy a brand new bike just for commuting just keep an eye out for local sales of decent quality 80 & 90’s Mtb’s and save yourself some money as well.